全文获取类型
收费全文 | 397篇 |
免费 | 43篇 |
国内免费 | 62篇 |
专业分类
航空 | 202篇 |
航天技术 | 242篇 |
综合类 | 28篇 |
航天 | 30篇 |
出版年
2024年 | 1篇 |
2023年 | 17篇 |
2022年 | 9篇 |
2021年 | 23篇 |
2020年 | 18篇 |
2019年 | 24篇 |
2018年 | 22篇 |
2017年 | 5篇 |
2016年 | 6篇 |
2015年 | 10篇 |
2014年 | 17篇 |
2013年 | 17篇 |
2012年 | 21篇 |
2011年 | 29篇 |
2010年 | 17篇 |
2009年 | 35篇 |
2008年 | 32篇 |
2007年 | 18篇 |
2006年 | 19篇 |
2005年 | 17篇 |
2004年 | 6篇 |
2003年 | 20篇 |
2002年 | 8篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 12篇 |
1997年 | 17篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 12篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有502条查询结果,搜索用时 375 毫秒
181.
In short wavelength case ultrasound propagation through woven carbon fiber-rein-forced composite (CFRC) materials essentially depends on their microstructure.Fiber bundle and weaving ply can be considered as basic structural elements of woven CFRC laminates.These structural units provide effective reflectors when ultra-short impulse high frequency ultrasound impinges on woven laminates.The reflection coefficient R0 achieves level of 3%-15% at ply-ply interface,0.2-0.4 in resin pocket region.This magnitude of reflection coefficient offers reasonable acoustic contrast to see packing,lay-up and resin pockets in woven laminate body by acoustic imaging methods.Microacoustic imaging technique is based on time-resolved echoes from reflections in woven CFRC laminates.A focal system with low aperture angle is used to generate 20-40 probe ultrasonic pulse wave.Its spatial resolution is up to 60mm at 50MHz in woven CFRC laminate.The results presented in this paper demonstrates the possibility and potentiality of characterization of microstrueture of woven CFRC laminates.Packing,lay-up,weaving nature,fiber bundle,resin pocket as well as their topology are exhibited clearly in the acoustic images by using microaeoustic technique. 相似文献
182.
Hong Jinsen 《中国航空学报》1996,(3)
TYPESOFFLOWOVERLEESIDEOFASLENDERTAPEREDWINGATSUPERSONICSPEEDHongJinsen(BeijingInstituleofAeronaulics,Beijing,China,100074)Abs... 相似文献
183.
184.
R.N. Manchester 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2709-2715
Recent pulsar surveys, especially those using the multibeam receiver on the Parkes radio telescope, have more than doubled the number of known pulsars, bringing the current total to about 1720. Of these, 171 have millisecond periods, 126 are members of binary systems and 99 are associated with globular clusters. Binary pulsars may be divided into several distinct groups depending on their observed properties such as pulsar period and age, binary period, eccentricity and companion mass. These groups all have different evolutionary histories. Correlations between the various properties are discussed using the increased samples now available. The Parkes Pulsar Timing Array is a project which has the goal of using timing observations of millisecond pulsars to make a direct detection of gravitational waves. The parameters and present status of the project are briefly discussed. 相似文献
185.
186.
We discuss the recent progress in studying the absolute and convective instabilities of circularly polarized Alfvén waves
(pump waves) propagating along an ambient magnetic field in the approximation of ideal magnetohydrodynamics (MHD). We present
analytical results obtained for pump waves with small dimensionless amplitude a, and compare them with numerical results valid for arbitrary a. The type of instability, absolute or convective, depends on the velocity U of the reference frame where the pump wave is observed with respect to the rest plasma. One of the main results of our analysis
is that the instability is absolute when U
l < U < U
r and convective otherwise. We study the dependences of U
l and U
r on a and the ratio of the sound speed to the Alfvén speed b. We also present the results of calculation of the increment of the absolute instability on U for different values of a and b. When the instability is convective (U < U
l or U > U
r) we consider the signalling problem, and show that spatially amplifying waves exist only when the signalling frequency is
in two symmetric frequency bands. Then, we write down the analytical expressions determining the boundaries of these frequency
bands and discuss how they agree with numerically calculated values. We also present the dependences of the maximum spatial
amplification rate on U calculated both analytically and numerically. The implication of the obtained results on the interpretation of observational
data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the
solar wind are convectively unstable in a reference frame of any realistic spacecraft. 相似文献
187.
近场散射数据的远场变换研究是具有发展前景的由近场测量目标,获取目标远场雷达截面的方法之一。根据近场获得的散射数据,外推获取远场的目标散射特性。主要是利用平面波谱展开,推导了近远场转换公式;以导体圆柱、平板等简单物体作为散射体仿真了近场散射数据的远场变换,证明了方法的可行性和正确性。 相似文献
188.
189.
时分多址(TDMA)扩频通信系统的同步捕获是一个很重要的问题,如何实现快速同步则更重要。采用声表面波器件,时间延时门和一致检测窗能十分满意地解决同步的快速捕获,精度达正负半个码元。本文还讨论了两种基本的快速同步捕获的方法,並给出了试验结果。 相似文献
190.
多年来,中小直径薄壁管超声水浸聚焦探伤法中,H=F-√R^2-L^2被称为最佳水声程公式,但探伤实践中发现,实际最佳声程和由此式计算出来的理论最佳水声程出入较大,甚至完全不符。其原因是什么?此式存在的条件是什么?其适应性怎样?最佳水声程的涵义是什么?应如何确定?本文从理论和实践两方面对上述问题进行了分析,并提出了相应的结论。 相似文献